Align specs to backtrace changes in promise.rb
Schedule promise dispatch only if EM is running

Just had a fun situation where next_tick
callbacks from reactor-less spec examples
where leaking into other examples with a
reactor.
Remove one level of deferring

We're already on a new stack when
the callback is called, no need
to schedule again - we can just go
ahead and fire it.
Add specs for .start_client and .start_server
Move Support::Handlers into a proper manager class
Give Promise some love

- Make #wait public
- Don't include Progress, it's default now
- Specs
Update mutation coverage percentage